Sulfuric acid (H2SO4) is the largest volume industrial chemical with approximately 176 million tons made worldwide in 2006. Most sulfuric acid is used to make phosphoric acid from phosphate rock, which in turn is used in fertilizer manufacturing. Additional applications are as a catalyst for alkylation in petroleum refining, chemical manufacturing, textile fiber processing, explosives manufacture, pulp and paper processing, inorganic pigments, detergents, ore leaching and metal pickling. (Müller, T. L. (2006) “Sulfuric Acid and Sulfur Trioxide,” in Kirk-Othmer Encyclopedia of Chemical Technology, John Wiley and Sons.)
Essentially all H2SO4 is now made by the contact process where sulfur dioxide (SO2) is oxidized to sulfur trioxide (SO3) using heterogeneous alkali-promoted vanadium oxide catalysts. The SO3 is then reacted with water to form H2SO4.
The oxidation reaction of sulfur dioxide:SO2+0.5O2=SO3 ΔHr×n=−99 kJmol−1  Eq. 1: Sulfur dioxide oxidation.is very exothermic, equilibrium limited, and exhibits a decrease in conversion with increasing temperature. To compensate for the equilibrium limitation, the sulfuric acid industry has employed a number of solutions: 1) increase the SO2 concentration in the feed, 2) increase the O2 concentration in the feed, 3) increase the number of catalyst beds with intermediate removal of SO3, 4) decrease the operating temperature of the catalyst beds, and 5) increase the pressure (Müller, T. L. (2006) “Sulfuric Acid and Sulfur Trioxide,” in Kirk-Othmer Encyclopedia of Chemical Technology, John Wiley and Sons). Virtually all sulfuric acid plants in operation today use multiple catalyst beds with heat removal and/or SO3 removal between the beds to drive the equilibrium toward SO3, see, for example FIG. 1 and FIG. 2. The need for higher conversion is not only driven by product cost, but also by limits on the amount of unconverted SO2 that can be released to the atmosphere. To meet current economic and SO2 tail gas emissions requirements, modern H2SO4 plants must limit their SO2 emissions to no more than 4 lb of SO2 per ton of sulfuric acid manufactured, which translates to 99.7% conversion. Most modern plants are designed to achieve lower SO2 emissions.
During the early years of the contact process, supported platinum (Pt) was used for oxidizing SO2 to SO3. The Pt-based catalysts had excellent activity, but were very sensitive to poisoning, especially from arsenic (As). Arsenic is especially problematic when roasting sulfide ores because of the presence of various minerals such as arsenopyrite (FeAsS). Because of the sensitivity of Pt toward As and other poisons, new catalysts based on vanadium pentoxide (V2O5) were developed (Twigg, M. V. (ed) (1989) Section 10.4 “Sulphur Dioxide Oxidation,” in Catalyst Handbook, 2nd ed., Manson Publishing). Current commercial SO2 oxidation catalysts are based on alkali metal promoted vanadium oxide (V2O5), particularly those promoted with cesium (Cs), potassium (K) and mixtures of alkali metals, supported for example on silica (SiO2) in various forms.
Existing commercial SO2 oxidation catalysts need to be operated near 400° C. to obtain acceptable reaction rates, which in view of the reversibility of the oxidation, limits the conversion that can be realized. To compensate for this, sulfur dioxide is currently oxidized to sulfur trioxide in a catalytic reactor (such as shown in FIG. 2) that contains multiple separate catalysts beds which are operated adiabatically, and where the inlet feed gas temperature is lower in each successive bed. Typically three (older plants), four, and sometimes five beds are used to further minimize SO2 emissions. In all of these SO2 converters intermediate gas cooling is used between the beds, so that the reaction temperature decreases in each successive catalyst bed. The last bed is operated at the lowest possible catalyst temperature that still gives acceptable reaction rates in order to maximize SO2 conversion. Even with the best current catalysts, however, reaction rates are not high enough at temperatures below 400° C. to meet increasingly stringent SO2 emissions regulations. As a result, current H2SO4 manufacturers operate the final catalyst bed at temperatures near or in some cases higher than 400° C. and simply scrub out unreacted SO2 from the plant's tail gas. Several patents specifically address these issues.
U.S. Pat. No. 3,259,459 relates to a process for production of SO3 in which SO2 is partially converted to SO3 in a first pass, SO3 is absorbed through an interpass absorption step and remaining SO2, from which SO3 is removed, is then passed into a subsequent catalyst bed at a temperature that is at least 20° C. lower than the preceding pass. A three stage process is specifically reported.
U.S. Pat. No. 3,963,423 relates to a high gas through-put process for the conversion of SO2 to SO3 in which the reactor includes a plurality of reaction chambers each having catalyst trays connected in parallel in the gas flow path.
There are also reports of methods and apparatus for oxidation of SO2 to SO3 where the feed composition or other process variables were changed to improve sulfuric acid output or economics. For example, U.S. 2003/0231998 reports methods and apparatus for oxidizing SO2 to SO3 where feed stream compositions are varied to improve the process. The published application provides descriptions of sulfuric acid contact plants and is incorporated by reference herein in its entirety for such description. U.S. 2007/0260072 relates to a process for SO2 oxidation to SO3 in which vaporized sulfur is provided in the gas stream containing SO2, SO3 and oxygen to enhance SO3 production. U.S. Pat. No. 7,361,326 relates to a process for production of sulfuric acid in which a strong feed (concentrated) gas containing 6 vol % to 30 vol % SO2 is used, which results in the final wet condensing stage having an acid dew point below 260° C. U.S. Pat. No. 7,704,476 relates to a process and plant for sulfuric acid production from SO2 in which the feed to the first contact stage contains more than 16 vol % SO2 with a volumetric ratio of sulfur dioxide to oxygen of more than 2.67:1. The process is reported to decrease the amount of gas that is passed through the reactor and to shift the thermodynamic equilibrium toward SO3. The contact gas can be supplied at pressures of 1-30 bar (preferably 3 to 12 bar). In all cases standard commercial catalysts (alkali-promoted vanadia) were either used, or the catalyst was not specified.
U.S. Pat. Nos. 1,941,426; 3,789,019; 3,987,153; 4,193,894; 4,431,573; 4,539,309; 4,680,281, and 4,766,104, all relate to vanadium-based catalysts for oxidation of SO2 to SO3. U.S. Pat. No. 1,941,426 relates to a method of making SO3 by oxidation of SO2 employing a cesium-promoted vanadium catalyst which may be supported on diatomaceous earth or silica. Conversion is reported to be improved in the cesium-promoted catalysts compared to the base vanadium catalyst particularly at temperatures between 450° C. and 375° C.
U.S. Pat. No. 3,789,019 relates to catalysts for oxidation of SO2 where the primary catalytic material is CsVO3 or RbVO3, a promoter, such as a metal sulfate, e.g., chromium potassium sulfate, and a carrier, e.g., refractory oxides, diatomaceous earth and/or colloidal silica. The catalyst is also reported to preferably include an activator selected from sulfates of cobalt, nickel or iron. In one example, a slurry of catalytically active components and carrier is dried, calcined and crushed to form 6-10 mesh granules. In another example, the slurry is dried to a paste and extruded into cylindrical granules which are optionally crushed to 6-10 mesh granules. In another example, a thick paste is formed by kneading catalytically active components and carrier after which the paste is dried, calcined and crushed.
U.S. Pat. No. 3,987,153 relates to a process for making sulfuric acid and reducing SO2 content in the off-gases of the process by scrubbing the off-gases with aqueous hydrogen peroxide (H2O2) and/or sulfur-containing peroxy acids. The patent refers to a multistage oxidation catalyst to convert SO2 to SO3, absorption of SO3 in water to form H2SO4 and the scrubbing process. The final oxidation stage of the process is reported to employ a supported CsVO3 or RbVO3 catalyst activated with cobalt or nickel sulfate and promoted with alkali metal sulfates, potassium aluminum sulfate or chromium potassium sulfate. Performance reported is at temperatures of 450° C. or higher.
U.S. Pat. No. 4,193,894 relates to a catalyst for oxidation of sulfur dioxide at temperatures above 300° C. which is a melt at reaction conditions and which includes sulfatized vanadium ions and two different promoters dispersed in an inert porous carrier. The promoters are reported to be (1) cesium ions optionally in combination with ions of another alkali metal and (2) ions of a metal which does not promote formation of inactive V4 species and the oxide of which has a heat of formation greater than 100 kcal/gram atom of oxygen and is at least partly soluble in the melt, such as aluminum, magnesium, yttrium or lanthanum. Pellets of catalytic material are reported to be formed by impregnating a porous carrier with a solution of certain active species or precipitating carrier with certain active species with carrier then being formed into pellets and dried.
U.S. Pat. No. 4,206,086 relates to the use of calcined and finely comminuted diatomaceous earth (a naturally occurring form of silica), particularly that from a certain fresh water diatom, as the support for alkali-promoted vanadium oxide. Extruded pellets of catalyst are reported to be prepared by dry mixing vanadium oxide, alkali sulfate and carrier and adding sufficient water to form a mixture for extrusion. Extruded pellets are dried, heated at 1000° C. and activated.
U.S. Pat. No. 4,431,573 relates to catalysts that contain V2O5 and alkali sulfate for oxidizing SO2 to SO3 in contact-catalysis systems. Reported catalysts are produced by impregnating prefabricated supports with solutions of active substances. The catalysts are reported to effect high conversion at a “low temperature.” Maximum catalytic conversion reported was at temperatures above 400° C.
U.S. Pat. No. 4,539,309 relates to catalysts for oxidizing SO2 to SO3 having a silica-based carrier with active material containing vanadium and alkali metal compounds. A specific catalyst preparation is described in which vanadium pentoxide is dissolved in an alkali solution, the solution is acidified by addition of sulfuric acid and the acidified solution is contacted with the carrier. The resulting mixture is molded, dried and calcined. In an alternate preparation, mechanically stable catalysts are reported to be formed by dissolving alkali metal silicate and/or silica sol in the alkaline solution prior to acidification and thereafter the solution is combined with the carrier.
U.S. Pat. Nos. 4,680,281 and 4,766,104 relate to a process for producing a catalyst for oxidizing SO2 to SO3 in which prefabricated carrier bodies are impregnated with solutions containing vanadium and alkalis. In specific examples, vanadium pentoxide, and certain alkali sulfates are combined with sulfuric acid to form an impregnating solution which is contacted with carrier. The catalysts are reported to be activated under oxidizing conditions at a temperature of 700 to 1000° C.
U.S. Pat. Nos. 5,175,136 and 5,264,200 relate to monolithic catalysts for converting sulfur dioxide to sulfur trioxide. The reported catalysts have platinum or alkali metal-vanadium active phases.
U.S. 2003/0157010 A1 reports a process for oxidizing SO2 to SO3 in a gas mixture containing 15-60 vol % SO2 which uses two catalyst layers. The first catalyst layer contains a commercially available vanadium pentoxide catalyst and the second catalyst layer contains a catalyst containing iron. The gas mixture is introduced into the first catalyst layer at temperatures of 350° C. to 600° C. The gas mixture is thereafter directed to the second catalyst layer with a temperature of 500° C. to 700° C. The second catalyst layer is reported to preferably contain 3 to 30 wt % arsenic oxide.
U.S. 2005/0287057 A1 reports SO2 oxidation described as “efficient high-volume oxidation” with vanadium or other catalytic dopants supported on activated carbon. The use of catalyst formulations using metals that can alternate back and forth between +4 and +6 oxidation states (such as tungsten or molybdenum), are reportedly able to increase efficiency. The gas containing SO2 is reported to be contacted with an activated carbon preparation containing catalytic metal dopant in a reactor that also contains an anhydrous liquid solvent. The solvent is described as a stripping agent to remove SO3 from the activated carbon preparation.
The present invention relates to SO2 oxidation catalysts that contain gold in the form of particles, including those that are a micron or less in size and possibly as small as a few nanometers. The following discussion relates to the various catalytic applications of gold including those in which gold is employed in nanoparticulate/nanocrystallite form.
Catalytic applications of gold are exemplified by its addition to a supported cobalt hydrodesulfurization (HDS) catalyst (Venezia A. M. et al. 2007 “Hydrodesulfurization Cobalt-based Catalysts Modified by Gold,” Gold Bull. 40(2)130-134) and its use when alloyed with palladium as a catalyst for vinyl acetate monomer synthesis (Bond, G. C.; Louis, C. and Thompson, D. T. (2006) Catalysis by Gold, Imperial College Press). Other applications for gold catalysts are typically low to moderate temperature reactions that include among others CO oxidation (and CO oxidation in H2 rich streams), total hydrocarbon oxidation, hydrochlorination of acetylene to make vinyl chloride, direct formation of hydrogen peroxide from H2 and O2, epoxidation of propylene to make propylene oxide, and the water gas shift reaction (Bond, G. C.; Louis, C. and Thompson, D. T. (2006) Catalysis by Gold, Imperial College Press). In the hydrodesulfurization (HDS) application, the gold is reported to be present in the catalyst as small particles (28, 37 or 60 nm). It was also reported that gold lowered the temperature required to reduce the Co3O4 (which contains a mixture of Co3+ and Co2+), to CoO and to metallic cobalt, and to thus keep the cobalt in a more disperse state. In vinyl acetate monomer synthesis, gold is reported to be alloyed with palladium, rather than being present in the catalyst as discrete particles. In vinyl acetate monomer synthesis, gold is reported to be alloyed with palladium, and act to isolate Pd sites and inhibit undesirable reactions (Chen, M.; Kumar, D.; Yi, C-W. and Goodman, D. W. (2005) “The Promotional Effect of Gold in Catalysis by Palladium-Gold, Science, 310 (5746) 291-293).
Unique catalytic properties are reported to be exhibited by gold nanoparticles supported on reducible metal oxides (Hutchings, G. J. (2005) “Catalysis by Gold,” Catalysis Today, 100, 55-61); Haruta, M. (1997) “Novel catalysis of gold deposited on metal oxides,” Catalysis Surveys from Japan. Vol 1(1) 61-73.) Typical examples of these catalytic properties are low-temperature catalytic combustion, low temperature CO oxidation (to form CO2), partial oxidation of hydrocarbons, hydrogenation of carbon oxides and unsaturated hydrocarbons, and the reduction of nitrogen oxides (Haruta, M., (2004). “Gold as a Novel Catalyst in the 21st Century: Preparation, Working Mechanism and Applications” Gold Bulletin, 37 (1-2), 27; Haruta, M., and Sano, H. (1983). Preparation of Catalysts III, Elsevier Science Publishers, B.V., Amsterdam; Haruta, M., Kobayashi, T.; Iijima, S, and Delannay, F. (1988). Proceedings of 9th Int. Congress of Catalysis, Calgary; Haruta, M., Kobayashi, T.; Sano, H. and Yamada, N. (1987). Chemistry Letters, Novel Gold Catalysts for the Oxidation of Carbon Monoxide at a Temperature far Below 0° C., pp. 405-408; and Haruta, M.; Tsubota, S.; Kobayashi, T.; Kageyama, H.; Genet, M. J. and Delmon, B. (1993) “Low-Temperature Oxidation of CO over Gold Supported on TiO2, α-Fe2O3 (hematite) and Co3O4 Journal of Catalysis, Volume 144, 175-192).
Bulk gold is reported to be essentially inert as a catalyst, but when gold nanoparticles are supported on or mixed with reducible metal oxides (such as Fe2O3 or TiO2), the resulting catalysts are reported to oxidize carbon monoxide (CO) to CO2 at temperatures well below 0° C. Extensive research on the use of nanoparticulate/nanocrystallite gold (Au), indicates that Au imparts its unique low-temperature catalytic behavior only when it is dispersed on a support such as a metal oxide or carbon at the nanometer scale, where it no longer exhibits the electronic structure of bulk gold; larger gold particles/crystallites appear to behave like the bulk noble metal and do not enhance low temperature catalytic activity (Hutchings, 2005; Bond 1999; Bond, G. C.; Louis, C. and Thompson, D. T. (2006) Catalysis by Gold, Imperial College Press; Haruta, M., (2004). “Gold as a Novel Catalyst in the 21st Century: Preparation, Working Mechanism and Applications,” Gold Bulletin, 37 (1-2), 27.)
U.S. Pat. Nos. 4,698,324; 4,839,327; 4,937,219; 5,051,394; and 5,789,337 of Haruta et al. are all related to methods of preparing gold-based catalysts. U.S. Pat. No. 4,698,324 reports an improved deposition precipitation method for depositing gold or a mixture of gold and a catalytically active metal oxide on a carrier in the presence of urea or acetamide to form a catalyst useful, for example, for efficient fuel combustion. The size of the gold particles/crystallites that were deposited is not discussed. U.S. Pat. No. 4,839,327 reports deposition methods to form “ultra-fine” gold particles on a metal oxide to generate a catalyst for reduction and oxidation, in sensor elements for flammable gases, and as an electrode catalyst. The methods reported involve (1) slow addition of an aqueous solution of a gold compound to an aqueous solution containing a metal oxide (pH 7 to 11); (2) addition of reducing agent to an aqueous solution of metal oxide and gold compound (pH 7 to 11); or (3) bubbling carbon dioxide gas into an aqueous solution of metal oxide and gold compound (pH 11 to 12) to deposit gold hydroxide on the metal oxide. The metal oxide with deposited gold hydroxide is then heated to convert gold hydroxide to metallic gold. The method is reported to provide gold particles of uniform particle diameter not exceeding 500 Å.
U.S. Pat. No. 4,937,219 reports similar methods for deposition of “ultra-fine” gold particles on alkaline earth metal compounds. U.S. Pat. No. 5,051,394 reports an ultra-fine gold particle-immobilized oxide produced by coprecipitation from an aqueous solution containing a gold compound, a water-soluble metal salt and a carboxylic acid or carboxylate. U.S. Pat. No. 5,789,337 relates to methods for forming gold nanoparticles of dimension less than 250 Å by contacting a support with evaporated gold compound.
There are several reports of Au-promoted V2O5 catalysts in the literature, but none for SO2 oxidation. For example, Au-promoted V2O5/SiO2 and MoO3/SiO2 catalysts have been reported to be active for the oxidation of C3 hydrocarbons and CO, gold was reported to make the V2O5 and MoO3 catalysts more easily reduced by the hydrocarbons or CO. [Ruszel, M.; Grzybowska, B.; Gasior, M.; Samson, K.; Gressel, I. Stoch, J. (2005) “Effect of Au in V2O5/SiO2 and MoO3/SiO2 Catalysts on Physicochemical and Catalytic Properties in Oxidation of C3 Hydrocarbons and of CO,” Catalysis Today, 99, 151-159.] The reference reports that the content of Au in the catalysts was 1 wt %.
In temperature-programmed reduction (TPR) experiments with hydrogen gas, Munteanu, G.; Ilieva, L.; Nedyalkova, R. and Andreeva, D. (2004) “Influence of Gold on the Reduction Behavior of Au—V2O5/CeO2 Catalytic Systems: TPR and Kinetic Parameters of Reduction,” Applied Catalysis, A: General, 277, 31-40 report that adding gold nanocrystallites (frequently referred to as nanoparticles in the literature) to V2O5/CeO2 catalyst made the V2O5 easier to reduce. In both cases the presence of gold lowered the activation energy for the equilibrium between vanadium (V) and vanadium (IV), i.e. V5+=V4++e−.
Studies of benzene oxidation over Au—V2O5/TiO2 and Au—V2O5/ZrO2, reported that gold lowered the activation energy for benzene oxidation, presumably by enhancing oxygen chemisorption [Andreeva, D.; Tabakova, T. and Idakiev, V. (1998) “Complete Oxidation of Benzene over Au—V2O5/TiO2 and Au—V2O5/ZrO2 Catalysts,” Gold Bulletin, 31 105-106.] The catalysts employed were prepared by deposition-precipitation of gold onto the support and impregnation with (NH4)2[VO(C2H4)2] with the atomic ratios Au:V2O5:MO2 (M=Ti, Zr) reported to be 1:1.3:31.
U.S. Pat. No. 6,825,366 relates to a process for epoxidation of olefins employing a catalyst comprising gold, preferably in nanometer size on a support material, where the support material contains one or more elements chosen from scandium, hafnium, vanadium, niobium, tantalum, chromium, molybdenum and tungsten and in which the catalyst is free of titanium. Specifically identified supports include those prepared by sol-gel synthesis employing silanol compounds, such as vanadium/tetraethylorthosilicate and which are described as vanadium silicate. It is also asserted that promoters including alkali metals can be added to the catalysts.
U.S. Pat. No. 5,567,839 relates to palladium/gold shell type catalysts for vinyl acetate production in which a barium salt is used to precipitate water-insoluble palladium and optional gold compounds onto a support prior to reduction with a reducing agent.
U.S. Pat. No. 6,468,496 relates to a supported gold-containing catalyst for producing hydrogen peroxide from direct liquid-phase reaction of hydrogen and oxygen. The patent refers to gold supported on a support such as titania, zirconia, titania-silica or zirconia-silica.
U.S. Pat. No. 6,136,281 relates to the removal of mercury from a stack gas by catalytic oxidation of elemental mercury to mercury(II)chloride (HgCl2). Gold-coated particular material is employed to oxidize mercury.
JP 2002305001 relates to an electrode catalyst containing platinum and gold carried on a conductive carbon material for use in a fuel cell.
U.S. Pat. No. 6,692,713 relates to preferential oxidation of carbon monoxide and/or methanol in a hydrogen-containing process stream employing a catalyst comprising gold on a support comprising non-reducible magnesium aluminum oxide in the form of MgAl2O4 spinel.
DE 10205873 A1 (also EP1478459 A1) relates to metal-oxide supported Au catalysts reported to have a narrow cluster size distribution and a high degree of dispersion for the Au cluster. The catalysts are reported suitable for the selective CO oxidation in reformer gases, the low-temperature water-gas shift reaction (WGS), the synthesis of methanol, the epoxidation of olefins, or the total oxidation of CO, hydrocarbons or halogenated gases.
JP 6039284 (published 1994) relates to a NOx decomposition catalyst having vanadium oxide and gold or a gold compound deposited on titanium oxide. The catalyst is reported to be employed to efficiently treat NOx-containing exhaust gas containing material that is poisonous to the catalyst such as water or SOx at relatively low temperature, such as 170° C. or lower.
JP 10216518 (published 1998) relates to a gold alloy catalyst useful as an exhaust gas purification catalyst. The alloy catalyst is made up of Au and one or more of Pt, Pd, Ag, Cu and Ni.
JP 4371228 (published 1992) relates to a catalyst to remove malodorous compounds such as aldehydes and ketones by oxidation. The catalyst is a metal oxide-gold mixture in which fine gold particles are fixed on a metal oxide, such as a p-type semiconductor oxide, e.g. cobalt oxide or nickel oxide or an n-type semiconductor oxide, e.g. iron oxide or titanium oxide and which is supported on a metal oxide carrier such as alumina or silica or a metal carrier such as stainless steel or iron.
U.S. Pat. No. 6,825,366 relates to catalytic epoxidation of olefins using oxygen and hydrogen employing a catalyst comprising gold, preferably in nanometer size, on a support material comprised of scandium, yttrium, lanthanum, zirconium, hafnium, vanadium, niobium, tantalum, chromium, molybdenum and/or tungsten and is essentially free of titanium. The catalysts are prepared by first mixing an oxide or other source of scandium, yttrium, lanthanum, zirconium, hafnium, vanadium, niobium, tantalum, chromium, molybdenum and/or tungsten with tetraethylorthosilicate (TEOS) and subsequently heat-treated to form a metal silicate. The catalysts are made by treating the metal silicate with a solution containing a gold compound and citric acid, which deposits gold particles onto the surface of the metal silicate. The catalyst is then washed, dried and calcined.
U.S. 2007/0093559 relates to oxidation or reduction of organic and inorganic compounds employing a supported catalyst consisting of nickel promoted with silver or gold which is present in an amount between 0.001% and 30% by weight based on the amount of nickel in the catalyst. The application refers to SO2 oxidation to SO3 employing the nickel catalyst. Gold and silver are reported to block active sites on the nickel catalyst such that the nickel shows a more “noble” behavior. Gold or silver is said to either block “poisonous side reactions” giving increased activity or reduce activity of the nickel catalyst. In the example of SO2 oxidation, a Au-promoted nickel catalyst sample is reported consisting of 17 wt % nickel and 0.3 wt % gold, prepared by incipient wetness of the support (spinel, MgAl2O4) using tetraammine gold (III) nitrate, (NH3)4Au(NO3)3, as the catalyst precursor followed by drying and calcining. The SO2 oxidation experiment was conducted at 380° C. with 0.7 vol % SO2 in the feed. The application reported a marked improvement in oxidation activity for both the silver promoted nickel catalyst and the gold promoted nickel catalyst compared to corresponding pure nickel catalyst.